Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building products; especially gypsum board. It is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded or otherwise formed to useful shapes. The base material from which gypsum board is manufactured is the hemihydrate form of calcium sulfate (gypsum), commonly termed stucco, which is produced by the heat conversion of the dihydrate from which the water phase has been removed.
The manufacture of gypsum products generally includes preparing a gypsum-containing slurry that contains gypsum and other components of the finished product, and then processing the slurry to remove the water and form and dry the remaining solids into the desired form. In one example of the making of a gypsum board, the gypsum slurry must flow onto a paper substrate. In a continuous process, the slurry/substrate combination is then sized by passing this combination between rollers. Simultaneous with this sizing step, a paper backing is positioned over the sized gypsum slurry. Accordingly, the gypsum slurry must possess sufficient fluidity so that a properly sized gypsum board can be made. Fluidity refers to the ability of the gypsum slurry to flow.
One undesirable trait of gypsum is that gypsum absorbs water, which reduces the strength of the products in which it is used and enables deleterious biological activity, such as the growth of mildew, mold, and other undesirable organism, to occur therein and thereon. Prior art products, like ordinary gypsum board, gypsum tile, gypsum block, gypsum casts, and the like have relatively little resistance to water. When ordinary gypsum board, for example, is immersed in water, the board quickly absorbs a considerable amount of water, and loses a great deal of its strength. Actual tests have demonstrated that when a 2 inch by 4 inch cylinder of gypsum board core material was immersed in water at about 70° F. (21.1° C.), the cylinder showed a water absorption of 36% after immersion for 40 minutes.
Previous attempts to provide water-resistant properties to gypsum board include incorporation of asphalt, metallic soaps, resins, and wax additives into a aqueous gypsum slurry. The resulting materials were difficult to use and the core properties difficult to control. Polysiloxane-based systems have also been used in attempts to impart water-resistance to gypsum board. However, the polysiloxane-based systems are both expensive and difficult to use. Polyvinyl alcohol has been used in an attempt to provide a room temperature system for use in adding water resistant properties to gypsum. However, the polyvinyl alcohol system tends to rapidly separate and thus typically requires continuous mixing prior to use, and there is also the potential for bacterial growth.
A finished gypsum product has also been coated with water resistant films or coatings. One specific example of a past attempt to provide a water-resistant gypsum product is the spraying of a molten paraffin wax or asphalt into an aqueous gypsum slurry. However, these coating applications add additional process complexity and machinery complexity to the manufacturing process as well as extending production times.
Additionally, existing products used to make wax emulsions for gypsum boards are available in less than sufficient amounts. For example, montan wax, made from lignite coal, is an existing product for use in wax emulsions for gypsum. However, montan wax is made only by a few manufacturers, and supplies and deliveries of materials have been observed to be insufficient for modern manufacturing process.
Accordingly, there is a need for a wax emulsion which is useful in imparting water-resistance to gypsum products, and which is economical to apply. There is a need for a alternative components for wax emulsions useful in imparting water-resistance to gypsum products. There also remains a need for an additive which is useful in imparting resistance to biological growth on gypsum products, and which is economical to employ.